The inventive subject matter generally relates to methods of powder metal processing and articles made therefrom. The techniques may also be used in forming parts from ceramic powders or ceramic and metal powder compositions. Certain embodiments of the inventive subject matter relate to methods of direct compression molding in conjunction with secondary forming operations. Certain embodiments relate to comolding or overmolding of parts using different powder composition to form different portions. Certain embodiments relate to powder metal processing to form implantable medical components.
Traditional metalworking techniques have been used historically to make medical components. The cost of machining medical components increases significantly when the parts have complex geometries. Complex geometries have many features with precise shapes and dimensional tolerances.
Metal injection molding (MIM) is a net-shape injection molding process for producing metal parts. Medical parts, such as orthopedic components made of forged wrought metal cost more than similar MIM components. Forging of complex shapes from wrought metal simple geometric shapes requires high forging forces and stroke lengths and/or multiple blows. Machining of forgings to net shape can result in excessive machining cost and metal yield loss if the forging is larger than it needs to be. MIM techniques are therefore being developed for the medical and other industries that need relatively small and complex parts. MIM operations are well suited for producing small, highly complex geometries in many different alloys including stainless steels, alloy steels, and custom alloys. Design and economic limitations of traditional metalworking operations, such as machining, can be readily overcome by metal injection molding. MIM operations are especially suited for producing net and near net articles with close tolerances, and have been used to produce articles for the agricultural, automotive, medical, small appliance, and sporting goods industries, to name just a few.
In conventional MIM operations, fine metal powders are mixed with a polymer binder system to form a feedstock suitable for injection molding. The injection molding process and equipment are similar to that used in the art of injection molding of plastics. The geometric characteristics of the powder particles have a significant impact on the packing density, size and shape of interstitial micro voids, bulk flow during injection, debinding behavior, and microstructure development during later sintering stages. Traditional MIM powders are finer than 25 microns in size; in a few circumstances, the particles may be as large as about 40 microns. The metal powder and polymer binder mixture is forced through a gate into a mold, the part cools in the mold and the molded part is then ejected, thus producing a “green” part having at least the near-final shape but larger size than the desired finished article. Conventional MIM design parameters generally limit the thickness of the largest section of the molded parts to about 12 mm due to dimensional instability, and internal molding related macro void formation. The economics of large MIM parts is generally unfavorable due to the high cost of very fine metal powders and the long time required to remove binders from parts with large sections.
After molding, the “green” part is debound to produce a “brown” part. During conventional thermal debinding, the part is subjected to one or more heating cycles to decompose, or otherwise remove, the polymer binder. Thermal binder removal can take a minimum of several hours and is fraught with the risk of introducing deleterious flaws such as bloating or blistering. Rapid heat is generally avoided, especially if the binder content of the part is high, since this can lead to considerable pressure buildup inside the part, which can lead to catastrophic bursting. After debinding, the now “brown” part is sintered to facilitate consolidation,” which results from metal-to-metal bond formation during sintering to create an interconnected mass of metal. During sintering, the part shrinks to its final size as a result of consolidation. The sintered part may be finished to the final shape using any number of processes, including but not limited to, coining, machining, grinding, cutting, polishing, or coating.
Conventional MIM operations are suitable for small parts with complex geometries to be made to net or near net shape in high volume. However, several technical difficulties arise when the part size increases or production volume decreases. For example, binder removal is a lengthy processing step that adds to the difficulty of MIM operations and becomes increasingly problematic as the size of the molded part, and hence the distance the binder must travel outwardly to be removed increases. Mold filling is also important so that empty corners, or other molding related macro voids, will not be left nor internal lamellar flaws introduced where the flowing composition layers join. Still further, handling, segregation, and bulk flow difficulties arise when providing larger quantities of feedstock during injection molding.
Thus, while MIM operations are capable of producing relatively small shaped articles in high volumes, these processes generally do not allow relatively larger parts (>12 mm in section thickness) to be made in a cost-effective manner. At lower production volumes, MIM operations are at a practical and economic disadvantage due to the relatively high cost of MIM molds. The as-sintered microstructure contains micro voids, and potentially, molding related macro voids that result in bulk densities less than or equal to 97% of theoretical alloy density. Microstructural features such as grain size are coarse and show little directionality when compared with wrought products. As a result, static and dynamic mechanical properties can approach but are not equivalent to the annealed properties of the wrought alloy.
Accordingly, it would be advantageous to have a cost-optimized process suitable for producing relatively large powder metal articles, of net or near net shape, having minimal internal voids in lower production volumes.
No matter what the size is, as-sintered parts produced from MIM and other pressure molding processes do not have the density, metallurgical structure, or mechanical properties provided by wrought materials for medical implant applications. Therefore, additional post-processing steps are needed to achieve wrought microstructures and properties.
Thus, there are a number of disadvantages and unfulfilled needs in the arts of MIM, and other metal forming techniques, such as forging, and casting. Among them are the following:                As-sintered powdered metal components processed by the cost effective MIM process do not have the density, metallurgical structure, or mechanical properties provided by wrought materials for medical applications, particularly surgical implant applications.        MIM traditionally has been limited to small parts (less than 12 mm thick) by the high cost of very fine metal powders and the difficulty in removing binders from parts with large sections.        Medical devices, such as implantable orthopedic components made of forged wrought metal cost more than similar MIM components.        Forging of near net shapes with complex geometries from wrought metal simple geometric shapes requires high forging forces and long stroke lengths and/or multiple blows.        Machining of forgings to near net shape can result in excessive machining cost and metal yield loss if the forging is larger than it needs to be.        Casting of large parts can result in shrinkage macro voids, microstructural segregation and coarse metallurgical structures.        Cast parts do not typically possess wrought mechanical properties.        Casting and forging of wrought materials do not easily allow for unitary parts or structures having zones with different metal compositions.        
In view of the foregoing problems and disadvantages, there is a significant need for improved powder-metal forming (“PF”) techniques to produce cost effective end products that are relatively large in size (versus traditional MIM) or complex in shape.